1. Field of the Invention
This invention relates to apparatus for viewing the magnetic pattern of a magnetized material, and in particular to a high sensitivity magnetic viewer for viewing low amplitude magnetic fields emanating from a weakly magnetized material.
2. Description Relative to the Prior Art
At least three ways of visually examining a magnetized body to determine its magnetic field distribution are known in the prior art. One way is to apply an aqueous solution of extremely fine iron particles directly to the surface of the magnetized material, and to observe where the particles congregate under the influence of the field originating in the material. This technique, known as the Bitter test, is a sensitive one, but it is untidy and attendantly inconvenient.
A second method utilizes a plastic sheet containing encapsulated Ni flakes dispersed in a liquid. The sheet is placed in contact with the magnetized surface, and the flakes rotate and align themselves with the magnetic field. Light reflected from the planes of the aligned flakes delineates the pattern of the magnetic field. This technique works well for strongly magnetized surfaces, but it will not image moderately strong magnetic layers such as conventional magnetic recording tape. This plastic sheet material is available under the trade name "Magneview".
The third method consists of a thin aluminum membrane stretched across the bottom of a small volume containing ferrite flakes dispersed in water. Using this device, magnetic tapes having low linear density recorded signals can be imaged, but those of moderate density are not imaged because the field from the tape at the surface of the ferrite particle layer is too small. A viewer incorporating this technique is available having the trade name "3M Viewer".
It will be noted that in the techniques practiced in the prior art, the magnetic field directly magnetizes and mechanically positions magnetic particles whose spatial distribution then replicates that of the field, allowing the field pattern itself to become visually discernable. When the field to be detected is too low in amplitude to effect mechanical aligning or positioning of the magnetic particles the techniques of the prior art fail. The present invention does not rely on particle alignment or positioning to provide a visual depiction of a low amplitude magnetic field.